refspec: move refspec parsing logic into its own file

In preparation for performing a refactor on refspec related code, move
the refspec parsing logic into its own file.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -3,6 +3,7 @@
 #include "config.h"
 #include "branch.h"
 #include "refs.h"
+#include "refspec.h"
 #include "remote.h"
 #include "commit.h"
 #include "worktree.h"